Tin surfaces are used in the manufacture of printed circuit boards, IC substrates, semiconductor wafers and related devices as a final finish, i.e., serving as a solderable or bondable surface for subsequent assembly steps. Tin is mostly deposited onto copper features of a substrate denoted as contact pads. The method of choice for this application is deposition of tin by electroless plating procedures with immersion plating as the most commonly applied method. The immersion plating process of tin or tin alloys—also called exchange reaction, cementation or displacement plating—onto a copper surface follows formula (1)Sn2++2Cu→Sn+2Cu+  (1).
The consequence of reaction (1) is that copper from the contact pad made of copper is dissolved during deposition of tin (The Electrodeposition of Tin and its Alloys, M. Jordan, E. G. Leuze Publishers, 1st Ed. 1995, p. 89-90).
The loss of copper during immersion tin plating can cause inacceptable failures in the manufacture of state of the art printed circuit boards (PCB) such as HDI PCB's (High Density Interconnect), IC substrates and semiconductor wafers which can have very thin or narrow copper contact pads to be coated with tin. Typical thickness or width values of contact pads of PCB's, IC substrates and semiconductor wafers are 50 μm, 25 μm, 15 μm or even less. Especially for contact pad dimensions below 25 μm the loss of copper during immersion tin plating has to be minimized and controlled. Otherwise, circuit interruptions and loss of copper pad adhesion to the substrate can occur.
The tin layer deposited onto a contact pad made of copper serves as a solderable and bondable surface for reflow and soldering processes as well as wire bonding. Tin layers for said applications typically have a thickness of ≦1 μm. On the other hand, a tin layer having a thickness of ≧1 μm or even ≧5 μm may be desirable. One possible application for this would be to serve as a solder depot for a successive soldering process. In such a case the corresponding loss of copper during immersion tin plating of a thin contact pad is not acceptable any more.
The amount of copper which constitutes a contact pad is even more reduced during reflow and soldering processes due to the formation of copper-tin intermetallic compounds (IMCs).
Höynck describes a process for deposition of thick tin-lead alloy layers by electroless plating onto contact pads made of copper (M. Höynck, Galvanotechnik 83, 1992, pp. 2101-2110). The loss of copper during deposition of the thick solderable layer is compensated by increasing the thickness of the contact pads by electroplating of copper prior to plating of the tin-lead alloy.
It is not possible to selectively deposit a thicker layer of copper by electroplating only where it is needed, i.e., onto the contact pads, since not all of the pads can be electrically contacted at this stage of the circuit board manufacture. Deposition of a thicker copper layer by electroplating in an earlier stage of the PCB manufacture or wafer metallization is not feasible because of restrictions in respect to achievable aspect ratios of successive copper etching steps.
Document US 2008/0036079 A1 discloses in the prior art section in paragraphs [0005]-[0007] a method for built up of a solderable contact pad in the manufacture of PCBs. The method comprises the steps of electroless plating of an adhesive layer, e.g. a tin layer, onto a copper contact pad. It is a disadvantage of the process that due to diffusion of copper the copper contact pad is decreased and a cavity is formed on the contacting site between the tin and copper (see Comparative Example 1 of the present invention).
Document US 2008/0036079 A1 further discloses in paragraphs [0025]-[0030] a specific embodiment of the invention for built up of a solderable contact pad in the manufacture of PCBs. The method comprises the steps of electroless plating of a copper layer onto a copper contact pad followed by immersion plating of an adhesive layer, e.g., a tin layer. The layer of copper plated with an electroless process serves as a reservoir for IMC formation during reflow and soldering operations. However, it is not the aim of said process that the copper layer deposited by electroless plating is completely consumed during immersion plating of the adhesive layer. The electroless copper layer should reduce the copper loss of the contact pad caused by formation of copper-tin IMCs during reflow and soldering processes. This process leads to an interface consisting of electroplated copper and electroless plated copper which is prone to form cracks after a reflow or soldering process, thus reduces the solder joint reliability (see Comparative Example 2 of the present invention).